International Journal of Science & Technology Volume 2, No 1, 75-81, 2007

75

Investigation of the Microstructure and Wear Properties of a Cast ZA Alloy

Zülküf BALALAN and Mehmet KAPLAN University of Firat, Faculty of Technical Education,

Department of Metallurgy, 23119 Elazığ, TURKIYE

mkaplan@firat.edu.tr

(Received: 09.10.2006; Accepted: 09.03.2007)

Abstract: In the present paper, microstructural changes and wear properties of a cast ZA alloy were studied

using scanning electron microscopy and dry sliding wear tests. Wear behaviour of the alloy was investigated on

a pin-on-disk wear tester under dry sliding conditions, rubbing against nitrided steel. Samples were tested at 80

N load and 190 rpm sliding velocity at room temperature. The decrease in wear loss was measured at an internal

of 50 m up to 1200 m distance. The results showed that the wear performance of certain specimens was superior

to that of others specimen, in that place which microstructure of specimen it was important function.

Keywords: Cast ZA alloy; Microstructure; Wear; Mechanical properties

Bir ZA Döküm Alaşımının Mikroyapı ve Aşınma Davranışlarının İncelenmesi

Özet: Bu çalışmada bir ZA döküm alaşımının mikroyapı değişimleri ve aşınma davranışları taramalı electron

mikroskobu ve kuru kayma aşınması testleri kullanılarak incelendi. Alaşımın aşınma davranışı kuru kayma

aşınması şartları altında pin-on disk aşınma testi ile ve karşı aşınma malzemesi olarak yüzeyi nitrürlenmiş çelik

kullanılarak gerçekleştirildi. Deney numunelerinin aşınma testleri oda sıcaklığında 80 N yük altında ve 190 dak -

1 kayma hızında yapıldı. Aşınma kaybı değişimleri 1200 m. mesafeye kadar ve her 50 m.de bir ölçüldü. Elde

edilen aşınma testi deney sonuçları, bazı numunelerin aşınma dirençlerinin daha yüksek bazılarının da daha

düşük olduğunu ortaya koymuştur. Bu aşınma direnci farklılığında mikroyapı değişimlerinin önemli bir

fonksiyonu olduğu anlaşılmıştır.

Anahtar Kelimeler: ZA döküm alaşımı; Mikroyapı; Aşınma; Mekanik özellikler

1. Introduction

Zinc based cast alloys, commonly referred

to as “ZA” alloys, has been developed during the

past year and is now increasing in commercial

usage. The chemical requirements for these

alloys are specified in ASTM B669 [1]. The ZA

alloys are suitable for casting by sand, permanent

mould, shell mould and high-pressure die casting

methods. These alloys exhibit mechanical

properties equal to or exceeding those of

conventional zinc die casting alloys and those of

cast iron, aluminium and copper alloys. In

addition, they have excellent bearing properties,

wear resistance and machinability. Advantage of

cast properties include low melting temperatures

and hence low melting energy consumption,

increased die life and mould stability. They can

be readily cast in thin sections in sand moulds

and require no fluxing [2].

Zinc-aluminium cast alloys have been

designated as ZA-8, ZA-12, and ZA-27. They

are aluminium contents range from 8 to 28 % for

ZA alloys, as indicated in literature. Aluminium

adding improves the fluidity and castability of

the molten ZA metal. On the other hand low

aluminium rate exhibits lower strength and less

dimensional stability than alloys containing

aluminium within the specified range [3].

Magnesium is added primarily to minimize

susceptibility to intergranular corrosion caused

by the presence of impurities, which is about

0.015-0.02 % Mg [4]. Copper, like magnesium,

minimizes the undesirable effects of impurities

which increases the hardness and strength of

castings. Range of copper for ZA alloys is 0.5 to

2.5 %. Nickel, chromium, silicon, and

manganese are not harmful in amounts up to the

solubility limit of 0.02 % Ni, 0.02 % Cr, 0.035 %

Investigation of the Microstructure and Wear Properties of a Cast ZA Alloy

76

Si and 0.5 % Mn [5]. Lead, cadmium and tin at

levels exceeding the limits cause die cast parts to

swell, crack, or distort, which may occur within

one year. The maximum limit for lead, which

can promote the occurrence of subsurface

corrosion, is 0.006 %. The maximum limit for

cadmium is about 0.005 %. Tin, like lead, can

promote subsurface corrosion, therefore is

restricted to the maximum safe limit of 0.005 %

[6].

Recent investigations have been focused on the

characterization of the ZA alloys and modified

versions such as corrosion, wear and other

mechanical properties. In other words,

mechanical properties, corrosion behaviours and

microstructures of these alloys, have been

carried out for different ZA alloys [7, 8, 9 and

10].

However, it can be said that there is not

much information available on modified version

of microstructure and wear properties of ZA

alloys. It is also appreciated that the

microstructure of ZA alloys, as it is true for any

alloy, is associated with various factors such as

compositions of alloy, production techniques

adopted etc., and that even a very small change

in one of these factors can seriously affect the

quality/performance of the material. Hence, this

leads to the argument that the field of

microstructure, phase formation and wear

properties of ZA alloys with different

compositions still remains open for investigation

for various purposes in industry.

In view of this, we have first of all produced

certain ZA alloys consisting of several

compositions such as Zn, Al, Cu, Si and Sn, and

then investigated the microstructure, mechanical

and wear properties by using scanning electron

microscopy (SEM) with Energy Dispersive X-

ray Spectrograph (EDXS) analyses, hardness,

tensile and wear test. The effects of wt. % Cu

and Si- additions on the microstructure and wear

properties were investigated.

2. Experimental procedure

ZA alloys were prepared from high purity

level by air-melting in an induction melting

furnace. Specimens were produced by casting

into metallic cylindrical moulds having 18 mm

diameter and 120 mm height. Five different

alloy compositions of specimens given in Table

1 were produced by casting process. Heat

treatment of the cast specimens was carried out

in an electrical furnace such that each specimen

was first solution-treated at 350 oC for 10 h., and

then quenched in air to provide the

thermodynamic conditions for the phase and

microstructure. The standard metallographic

techniques were used for scanning electron

microscopy (SEM) studies. The microstructure

of the specimens shown in Fig. 1, which were

prepared by a disc-jet technique; samples for

microstructural studies were polished and etched.

The reagent for etching used was a solution

containing 5 g CrO3, 0.5g Na2SO4 and 100 ml

H2O for 10-15 minutes. To ensure

reproducibility of results, five specimens were

used for each composition. Hardness of the as-

cast ZA alloy samples measured with a Vickers’s

hardness tester by applying a load of 50 g. The

data of hardness reported in this study represents

an average of 8 measurements.

Dry sliding wear behaviour of these alloys

was investigated on a pin-on-disk wear tester

under dry sliding conditions, rubbing against

nitrided steels. The polished wear specimens

were tested at 80 N load and 190 rpm disc

velocity at room temperature. SAE 7140 nitrided

steel material was chosen as a counterface and its

hardness of ∼310 HV, and 60 mm diameter. The

wear loss for each specimen was measured at an

internal of 50 m up to 1200 m distance. The tests

were repeated four times for each specimen. The

results of weight loss were measured by a

sensitive of a 10-4

scale. The total value of weight

loss was calculated up to a 1200 m distance.

Balalan and Kaplan

77

Table 1. Tensile strength, hardness value and chemical compositions of the specimens

Elements (wt. %) Symbols

of specimens

Tensile strth.

(MPa)

Hardness

(HV) Al Cu Si Sn Mg Pb Zn

S1 282 105 20.93 0.98 0.09 0.40 0.011 ≤0.004 Rem.

S2 387 234 25.40 1.12 1.15 0.50 0.020 ≤0.004 Rem.

S3 395 248 25.90 1.10 2.40 0.52 0.013 ≤0.004 Rem.

S4 368 235 26.70 1.10 1.17 0.50 0.018 ≤0.004 Rem.

S5 391 267 37.40 3.15 1.72 0.39 0.014 ≤0.004 Rem.

Investigation of the Microstructure and Wear Properties of a Cast ZA Alloy

78

Fig.1. SEM images and their different regions of S1, S2, S3, S4 and S5

3. Results and Discussion 3.1. Microstructure of the alloys

The SEM micrograph and surface analyses

(EDXS) of certain regions of the specimens (S1-

S5) are given in Fig.1.S1-S5 respectively. When

the EDXS analysis results are compared with Al-

Cu-Zn, Cu-Zn and Al-Zn-phase diagrams given

in ref. [6], the following results can be said on

the SEM micrograph’s results for specimens;

Fig.1.S1 shows the microstructure results of S1,

as seen on the micrograph, it consists of Zn-22Al

alloy and fine-grained structure. The results of

S2 given in the same Fig. shows that the addition

of Cu and Si led to the formation of Si based on

Zn+Al and CuZn5 (ε) phases in main matrix of

α-Zn+β-Al. The influence of Cu and Si contents

on the microstructure of S3 is given in it. In this

microstructure of specimen it is obvious that the

increase of Si content causes the formation of

Si+Zn phases in main matrix and an increase in

hardness value. The results of S4 are given in it,

which is seen the specimen has a main matrix of

α+η and τ′ phase, which is a distinct phase

separated as Zn and Al-rich, has also a relatively

narrow range of composition surrounding, and it

consists of 53.98 wt. % Cu, 15.89 wt. % Zn and

30.13 wt. % Al. The τ′ phase is isomorphous and

that has a deformed structure based on an

ordered body-centred-cubic lattice [6], so it has

been increased the hardness of specimen S4.

The results of the last specimen are given in

Fig.1.S5. As seen in the grey regions belong to

α+ η main matrix, the dark grey colored images

are distributed along the grain boundaries, which

belong to Si+Zn compounds. The whitish

colored images and light white regions on the

SEM picture correspond to CuZn5 (ε) phases. It

can be sad that, the general microstructure

structure all of the specimens is the near-

eutectoid ZA and near-monotectoid ZA alloys

consisted of aluminium-α dendrites and zinc

interdendritic phases.

3.2. Hardness and tensile strength The tensile strength and hardness value results

of specimens given on Table 1, it is showed that

the addition of Cu and Si caused increase of the

specimen’s hardness from 105 to 267 HV. This

increase is due to some important phases such as

aluminium-rich τ′ phase and Si+Zn, which were

distributed in main matrix α-Zn +β-Al, so that

the hardness increased. The results of the tensile

data obtained from the specimens are also given

in Table 1. It can be said that tensile strength

increased from 282 to 391 MPa, which is

significantly with increasing ratio of Al, Cu, and

Si, as well as the mechanical properties of ZA

alloys were affected by the addition of Al, Cu

and Si as known in literature [3, 8, and 10].

3.3. Wear properties

The worn surfaces of the specimens are

shown in Fig.2. Worn surface of S1, S2 and S4

Balalan and Kaplan

79

was not smooth, which occurred a higher wear

loss with deeper wear groves and entrapped

debris during wear test. But the same damage

didn’t occur on the wear surface of S3 and

especially S5. In other words, no much swelling

and wide deep wear appeared on the wear

surface of S3 and S5. We should also note that

some deep pits/craters and continuous scratches

were observed on the wear surfaces of specimens

S1, S2 and S4 as seen in the Fig.2.

In Fig.3 wear loss versus applied a constant of a

80 N load and 190 rpm disc velocity at room

temperature for ZA alloy pins, are shown, it is

seen that the influence of adding from 0.09 up to

the 2.40 wt % Si in the S3 specimen and from

0.98 up to the 3.15 wt % Cu in the S5 specimen

are resulting of CuZn5 (ε) and τ′ phases on the

wear loss of the specimen pins. We note that the

wear losses of the specimen were measured as a

function of sliding distance. As seen in the figure

that the wear loss of S3 and S5 specimens is

considerably lowers than that of S1, S2 and S4.

In this way, the highest wear resistance was

obtained from S3 and S5 specimen while the

others have lowest wear resistance. This

probably, the worn phase particulars of S3 and

S5 specimens were remain considerably on the

worn surface, while the phase particulars on the

surfaces of the other specimens (S1, S2, and S4)

were broken out without wearing during wear

tests processes.

Investigation of the Microstructure and Wear Properties of a Cast ZA Alloy

80

Fig.2. Worn surface of specimens S1, S2, S3, S4, and S5 and pins tested at

0

5

10

15

20

25

30

0 200 400 600 800 1000 1200

Sliding distance (m)

Wear

loss, m

m 3 /

Nm

x 1

0 -2

S1 S2 S3 S4 S5

Fig.3. Wear loss versus applied a constant of a 80 N load and 190 rpm disc velocity at room temperature for ZA

alloy pins.

4. Conclusions

Investigation of the microstructure and wear

properties of a cast ZA alloy, which was

solution-treated at 350 oC for 10 h., investigated

by SEM with EDXS, tensile strength, hardness

and a pin-on-disk wear tester techniques can be

concluded as follows;

1. ZA alloys without Cu and Si elements have

not enough mechanical and microstructural

properties.

2. The results of SEM micrograph showed that

some important phases particulars and

compounds such as aluminium-rich τ′ phase and

Si+Zn formed and distributed in main matrix of

α-η, as a result of addition of elements such as

Cu and Si, and so that the tensile strength and

hardness increased specimens.

3. In general, the higher wear resistance was

obtained from S3 and S5 specimen than the

others, so deep pits/craters and continuous

scratches were observed on the wear surfaces of

specimens S1, S2 and S4. It is probably, due to

the particulate of CuSn5 (ε), τ′ Si+Zn phase and

compounds has broken out from the body of pins

without wearing during wear tests processes.

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81

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